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March 9, 1965 M. M. MARKOWITZ PROPELLANT COMPOSITIONS CONTAINING STABILIZED AMMONIUM PERCHLORATE Filed Oct. 12 1962 I 300 TEM PERATUR E PURE NH4CLO4 IN AIR 5 INVENT M EYER M. MARKOW l T BY ATTYS R A m 1 0 A U E 1 T R M I U I E P I W l M T E W J T O -w 4 w I I H i N O L I M l O -m N19 United States Patent 3,172,793 PROEELLANT (IGMPQSITEQNS CGNTAENENG STABTLEZED AMYMQN Meyer M. Marlrowitz, Ardnrore, Ra, assignor to Foote Mineral Company, Philadelphia, Pin, a corporation of Pennsylvania Filed Get. 12, 1962, Ser. No. 2.302% 12 Claims. (Cl. 149-19) This invention relates to a process for improving the stability of ammonium perchlorate and to ammonium perchlorate-containin g compositions having improved stability. More particularly, this invention is directed to a process whereby the thermal stability of ammonium perchlorate is improved and to mixtures containing ammonium perchlorate having improved thermal stability.

There is at present a considerable demand for ammonium perchlorate of high purity for use as an oxidizer in solid rocket propellants and various explosives. Ammonium perchlorate oxidizing agents are particularly attrative in propellants and explosives because of their low hygroscopicity and high density, the latter property resulting in propellant compositions having attractive density and bulk strength properties.

Ammonium perchlorate, however, is inherently unstable at elevated temperatures. This undesirable stability characteristic has severely limited the range of utility of materials containing ammonium perchlorate to fairly low temperature applications. It is extremely important that propellants maintain a predetermined burning rate; this rate is in part dependent upon accurately controlling the stability of the oxidizer employed. It is further essential that the ballistic potential of a propellant be maintained within critical limits even during rolonged exposure to extreme variations in thermal conditions. However, composite solid propellants containing an ammonium perchlorate oxidizer have been susceptible to decomposition during storage when exposed to rises in temperature. Similarly such compositions tend to decompose during use when exposed to temperature rises due to aerodynamic heating. Such decomposition of the ammonium perchlorate oxidizer results in changes in the ballistic properties of the composition. For example, ballistic deficiencies such as decreased total energy, sporadic burning rates and the like result which tend to produce propellant compositions having unreliable ballistic properties.

Unless precautions are taken in manufacturing and storing ammonium perchlorate, the crystals thereof will generally contain water, either occluded therein or absorbed thereon. This water tends to cause caking of the product during storage and processing difiiculties during manufacture of ammonium perchlorate containing compositions. For example, the presence of Water in ammonium perchlorate crystals tends to cause blocking of the orifices of pulverizing and sizing equipment used to comminute the oxidizer to the desired particle size, thus impeding control of the particle size. Moreover, solid propellant compositions usually contain polymerizable components and any water associated with the ammonium perchlorate would interfere with such polymerization. Of course, the presence of water in any form in a solid propellant represents an unattractive weight addition to the propellant.

Therefore, it is accepted practice in the propellant and explosive industry to subject ammonium perchlorate compositions to relatively low temperature drying processes wherein drying conditions below the decomposition temperature of the ammonium perchlorate are maintained for prolonged periods to produce a substantially waterfree crystalline product suitable for use in explosives and propellants.

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It is known that certain compounds which are present during the production of ammonium perchlorate, often are retained in the finished product. It is further known that when these compounds such as perchloric acid and potassium perchlorate are present in the ammonium perchlorate product in even trace amounts, they have a deleterious effect on ammonium perchlorate, particularly with respect to its thermal stability. Additionally, certain low melting point, inorganic compounds, such as lithium perchlorate, adversely afiect the thermal stability of ammonium perchlorate by forming a low melting point liquid phase. E tectic mixtures of low melting point inorganic compounds and ammonium perchlorate have considerably less thermal stability than the pure ammo nium perchlorate. Therefore, if ammonium perchlorate is exposed to temperatures near its decomposition temperature it is essential that it be essentially free from such impurities and low melting point inorganic compounds to avoid decomposition. In order to avoid such effects, exhaustive purifying processes are employed to produce an essentially pure ammonium perchlorate product.

Composite solid rocket propellant grains normally consist of at least one solid oxidizing agent such as ammonium perchlorate uniformly distributed throughout a matrix of a fuel-binder material. The propellant grains are generally made by mixing the solid oxidizing agent, and other particulate materials used to enhance the ballistic and physical performance of the product, with a liquid linear polymer binder. Generally, the propellant mixture is solidified by curing the polymer binder at a temperature below the decomposition temperature of ammonium perchlorate. Even in the presence of various curing promoters such as cross-linking agents and the like, this curing process is time consuming and diflicult because of the low operating temperature restrictions imposed on the process by the thermal instability of the ammonium perchlorate oxidizing agent present.

It is therefore an object of this invention to provide a process for the stabilization of ammonium perchlorate at elevated temperatures.

It is also an object of this invention to provide an improved composition containing ammonium perchlorate, said composition being stable at elevated temperatures.

It is another object of this invention to provide a process whereby ammonium perchlorate and mixtures containing ammonium perchlorate can be stored and used at relatively high temperatures without adversely affecting the stability of the ammonium perchlorate or of the mixture in which it is contained.

A further object of the invention is to provide a propellant containing an ammonium perchlorate oxidizer which can be used and stored without degradation throughout a fairly broad temperature range.

Still another object of the invention is to provide an improved method of drying ammonium perchlorate.

It is also an object of the invention to provide an ammonium perchlorate composition which can tolerate various impurities while maintaining a satisfactory thermal stability.

Another object of the invention is to provide an improved method of curing compositions containing ammonium perchlorate.

Other objects of the invention will become apparent from a consideration of the following specification, drawing and the claims.

It has been discovered that the thermal stability of ammonium perchlorate at elevated temperatures can be unexpectedly improved by mixing with the ammonium perchlorate a compound capable of producing an atmosphere of ammonia at a temperature below the decom:

position temperature of the ammonium perchlorate. That is, it has now been discovered that ammonium perchlorate, propellants, and other compositions containing ammonium perchlorate can be stabilized at high temperatures without degradation of the ammonium perchlorate by providing an atmosphere of ammonia, said gas obtained from the decomposition of certain substances having a decomposition temperature below that of ammonium perchlorate.

Examples of solid compounds, which decompose, forming ammonia gas at a temperature below about 200 C. include ammonium carbonate, ammonium bicarbonate and urea. However, not all compositions capable of producing ammonia produce a stabilizing effect on ammoniurn perchlorate at elevated temperatures, for example ammonium nitrate, ammonium chloride and ammonium bromide do not have satisfactory decomposition temperatures and do not stabilize ammonium perchlorate at elevated temperatures.

In a preferred embodiment the solid compounds which are capable of producing ammonia at the requisite temperature are used in a closed system, wherein the stabilizing ammonia gas once produced is retained within the confines of the system.

It is understood that the stabilizing compounds suitable for use in a closed system can have decomposition temperatures substantially below 200 C. The decomposition products of these stabilizers will be retained in the closed system to impart their stabilizing effect on the ammonium perchlorate at elevated temperatures.

It has been found that the unexpected thermal stabilizing effects of the stated compounds are apparently limited to ammonium perchlorate. That is, other perchlorates used as oxidizing substances, for example, potassium perchlorate and sodium perchlorate which are similarly sensitive to elevated temperatures substantially decom pose at their decomposition temperature in the presence of the compounds in open and/ or closed systems.

It has further been discovered that the stabilizing compounds impart an unexpected stabilizing effect to am monium perchlorate compositions containing impurities. That is, it has been found that the adverse effects on the thermal stability of ammonium perchlorate compositions produced by certain impurities such as perchloric acid and lithium perchlorate which were discussed above, are unexpectedly suppressed by the instantly described stabilizing compounds. Such impurities can be tolerated in ammonium perchlorate compositions which are subjected to high temperatures if the composition is maintained in the presence of the stabilizers, therefore, extensive purifying and processing of ammonium perchlorate is rendered unnecessary.

The present invention can advantageously be utilized to remove water from water contaminated ammonium perchlorate by means of high-temperature drying meth ods, while avoiding the hazard of decomposition of the oxidizer. Thus, the rate of drying ammonium perchlorate can be increased significantly. Moreover, the increased thermal stability achieved by the stabilizers of the invention can be utilized in producing ammonium perchloratecontaining explosives wherein the oxidizer is included in the explosive grain during curing of the liquid polymer fuel composition. By means of the present invention curing of the liquid polymer fuel can be achieved at significantly higher temperatures.

The present invention will be more readily understood from a consideration of the drawings in which:

FIGURE I is a conventional differential thermal analysis curve obtained with pure ammonium perchlorate under an atmosphere of flowing air; and

FIGURE II is a differential thermal analysis curve obtained with a mixture of ammonium perchlorate and urea.

Mixtures of the solid stabilizing agents could be employed, and mixtures of such solids with ammonia gas could be combined in either open or closed systems to produce satisfactory stabilizing results. That is, for example, a mixture of ammonium carbonate, urea and ammonia gas in a closed system would have satisfactory stabilizing properties.

The only significant limitation on the amount of ammonia producing composition compound employed is that the amount of ammonia gas produced thereby be sufficient to effect thermal stability of the ammonium perchlorate at elevated temperatures. On the other hand, excessive amounts of such gas producing agents are not required and would not be employed particularly in explosive and propellant compositions.

Forming the ammonia gas in situ in a closed system is particularly attractive in certain solid propellant compositions and explosive compositions having a relatively high density.

Forming the ammonia gas in situ eliminates the need for diffusional passages and the like, for if such a gas producing compound is intermixed throughout the propellant, upon decomposition the ammonia gas produced would be entrapped in the propellant grain itself, thereby exerting its stabilizing effect on the ammonium perchlorate present in the immediate area defined by the propellant grain. The propellant grain therefore would serve as the container for the gaseous ammonia, exerting the necessary mechanical forces to retain the gas within the grain. Although the ammonia producing stabilizing compounds can be employed in open systems, loss of some of the ammonia gas produced cannot be avoided, thus rendering this approach economically less attractive.

The rate of flow of the ammonia produced by decomposition of the ammonia producing compound will be sufficient to maintain a partial pressure of ammonia sufficient to stabilize the ammonium perchlorate at the environmental temperature. For example, the rate of flow of the ammonia can range from about 1 to about 500 cc. (measured at room temperature and atmospheric pressure) per 1% g. or" amomnium perchlorate per minute. Preferably the rate of flow of the ammonia gas is from about 5 to about 100 cc. per 100 g. ammonium perchlorate per minute.

Ammonium perchlorate is usually prepared by reacting sodium perchlorate and ammonium chloride in aqueous solution to produce ammonium perchlorate and sodium chloride. Ammonium perchlorate has a temperature gradient of solubility that is substantially greater than that of sodium chloride and this difference in solubility has been utilized in a number of known processes for separating these two products. Usually a cyclic process is employed and often multiple separation steps or even fractional crystallization are required to produce an ammonium perchlorate product having the requisite purity.

The composite propellant explosives containing ammonium perchlorate are generally composed of from about 10 to about 30% of a linear polymeric binder and a curing agent and from about 70 to about ammonium perchlorate oxidizing agent, ballistic modifiers and other suitable additives necessary to achieve the desired performance of the propellant or explosive. When the propellant contains less than about 10% matrix, it has undesirable physical properties, exhibits deficiencies in physical strength and is difficult to process. The physical properties of the propellant improve as the matrix content is increased. However, when this value exceeds about 30%, the propellant again exhibits unsatisfactory ballistic properties such as lack of strength to withstand field handling, acceleration, and the like and difficulty in ignition, decreased total energy, smoke or carbonaceous exhaust products, etc.

In addition to ammonium perchlorate, any solid ino r ganic or organic oxidizing agent compatible with ammonium perchlorate and thermally stable under the conditions imposed and capable of reacting with the matrix and other fuel ingredients in combustion processes can be used. Moreover, by means of the present invention the use of an additional oxidizing agent such as lithium perchlorate, ammonium nitriate and potassium perchlorate can be tolerated within certain concentration limitations even through these materials heretofore had adverse elfects on the thermal stability of the ammonium perchlorate. That is, it will be recalled that the stabilizing compounds depress the adverse effects such oxidizing compounds have on ammonium perchlorate thereby allowing their use as oxidizing agents along with ammonium perchlorate or, for that matter, as impurities in the ammonium perchlorate. In addition, certain organic derivatives of nitric acid, hydrogen peroxide, chromic acid and perchloric acid can be tolerated due to this depressing effect of the herein defined stabilizing compounds. The propellant may also contain various components to enhance its ballistic properties, thus various oxidizer decomposition catalysts can be employed as well as materials such as ammonium oxalate and the like which serve to cool the gases generated by the propellant. Moreover, the composition can be provided with different ballistic potential by the incorporation of carbon black, copper chromite, ferrocene or other suitable rate catalysts. The propellant can also contain finely divided metallic components such as aluminum, magnesium, berryllium and the like.

Composite, solid propellant grains normally consist of ammonium perchlorate uniformly distributed through a matrix of fuel binder material. The propellant grains are usually made by mixing the solid oxidizing agent and other solid additives which may be present to enhance the ballistic and physical performance of the product with the liquid matrix which is solidified after uniform dispersion of the solid materials has been obtained. The liquid linear polymer binder employed for the matrix of the propellant grain includes those polymerized products resulting from the reaction of one or more types of monomers and in the stricter sense, copolymers of two monomers, terpolymers of three monomers, etc. Usually these liquid linear polymer binder materials have active centers for cross-linking and curing of the linear polymers is effected in the presence of the various cross linking agents reacted with these active centers. The cross-linking is accomplished by a reaction of a bifunctional compound with active centers in the linear chain polymer and is accompanied by marked increase in the viscosity of the solution. Polymers particularly well suited to use in developing the matrix of the propellant include the class of polymers derived from unsaturated carboxylic acids of the acrylic acid type such as acrylic acid, methacrylic acid and vinyl acrylic acid. Polymers derived from various ethylenically unsaturated olefins and diolefins such as styrene, acrylonitrile, methacrylonitrile, vinylidene chloride, butadiene, isoprene, 2,3-dimethylbutadiene, chloroprene and the like are particularly preferred a as source of fuel. In addition to these polymers, various other materials can be used in forming the matrix, for example, various polysulfidm and urethanes are often employed. Other materials such as nitrocellulose, polyesters, polyethers and various polyamides can also be satisfactorily employed as a matrix for the propellant or explosive compositions.

When a closed system is employed it is understood that the vessel containing the ammonium perchlorate contained in the closed system should be capable of withstanding the pressures imposed thereon either initially or by the in situ formation of the stabilizing ammonia gas. Various conventional pressure vessels capable of withstanding such pressures as are encountered in the instant invention would be suitable and are well known in the art.

The differential thermal analysis method referred to in the drawings is a convenient method used for determining the thermal stability of substances such as ammonium perchlorate. According to this method, the sample under study is heated simultaneously with, but separately from, a material known to undergo no thermal changes over the temperature range. That is, a substance which is thermally inert and undergoes no physical or chemical changes over the temperature range of interest. Such thermally stable substances as aluminum oxide, potassium chloride and sodium chloride are often used as reference materials in differential thermal analysis. During heating, the temperature of the ammonium perchlorate is measured continuously as Well as the temperature difference between the ammonium perchlorate and the inert reference material. By plotting the results of heating the two samples, a curve of sample temperature versus temperature difference is obtained, where changes in the slope and discontinuities of the curve indicate the occurrence of chemical or physical changes in the ammonium perchlorate. Thermal decomposition of ammonium perchlorate proceeds by virtue of an exothermic oxidation/reduction reaction and therefore readily lends itself to study by the differential thermal analysis method.

The present invention will be more readily understood from a consideration of the following examples which are given for the purpose of illustration only and are not intended to limit the scope of the invention in any way.

Example A This example illustrates the thermal instability of am monium perchlorate at elevated temperatures under an atmosphere of flowing air.

One gram samples of ammonium perchlorate and aluminum oxide are separately, but simultaneously, heated from about 50 C. to about 500 C. in the presence of 50 cc./min. of flowing air. The aluminum oxide reference material exposed to similar conditions is essentially inert throughout this temperature range. Therefore, any changes in the slope of the differential thermal analysis curve would indicate endothermic or exothermic changes occurring in the ammonium perchlorate material.

FIGURE I is the differential thermal analysis curve obtained for ammonium perchlorate by means of the differential thermal analysis method described above. It is apparent from the curve in FIGURE I that at about 240 C. an endotherm corresponding to a crystallographic transition occurs, immediately followed by an exothermic break. Decomposition is evident at this exothermic break by the condensation of liquids and solids above the cooler portions of the sample test tube. The exotherm was found to peak at about 307 C. Between about 310 C. and about 400 C. there is a gradual change in the temperature differential value and at about 400 C. a large exotherm starts. Upon the completion of this exotherm, the decomposition of the ammonium perchlorate was complete.

The following example illustrates the differential thermal analysis obtained for a mechanical mixture of ammonium perchlorate and urea.

Example B A mechanical mixture of weight percent ammonium perchlorate and 10 weight percent urea is subjected to thermal analysis. It is found that the heating together of the components of this mixture results in the internal generation of ammonia gas and the consequent increase of thermal stability of ammonium perchlorate. This result is illustrated in FIGURE II of the drawing wherein it is evident that no exotherm occurs following the endotherm at 240 C. and it is also noted that no condensate or liquid is observed above the sample. Only at about 300 C. is there evidence of the onset of decomposition as seen by the presence of condensate. However, decomposition becomes rapid at about 380 C. as evidenced by the small exotherm and finally at about 400 C. the large exotherm starts to form with subsequent complete decomposition of the ammonium perchlorate content.

7' The following examples (C and D) illustrate the stabilizing effect of ammonium carbonate on ammonium perchlorate at elevated temperatures in a closed system.

Example C Ammonium perchlorate (50 grams) is placed on a glass liner in a 500 cc. stainless steel bomb. The bomb is then heated and maintained at a temperature of 200 C. for 16 hours. During this period a substantial increase in pressure in the bomb is observed. This increase in pressure is attributed to the accumulation of ammonium perchlorate decomposition products such as oxygen, nitrogen, nitrogen oxides, hydrochloric gas, water vapor and the like.

Example D An intimate mixture of ammonium perchlorate (50 grams and ammonium carbonate (2 grams) is placed on a glass liner in a 500 cc. stainless steel bomb. The bomb is heated and maintained at a temperature of 200 C. for 16 hours. No substantial increase in the total pressure is observed beyond that attributable to the presence of ammonium carbonate; thus indicating that the ammonium perchlorate is retained in a condition substantially free from decomposition.

It is further understood that ammonium bicarbonate can be used in a closed system to produce results comparable to those achieved in Example D.

From the foregoing description it will be evident that numerous variations and modifications can be made in the present process without departure from the invention.

The claims:

1. A process of improving the thermal stability of compositions containing ammonium perchlorate which comprises mixing with said ammonium perchlorate at least one solid compound capable of decomposing into ammonia at a temperature below 200 C. selected from the group consisting of ammonium carbonate, ammonium bicarbonate and urea.

2. The process according to claim 1 wherein said compound comprises ammonium carbonate.

3. The process according to claim 1 wherein said compound comprises ammonium bicarbonate.

4. The process according to claim 1 wherein said compound comprises urea.

5. An ammonium perchlorate-containing propellant composition having improved thermal stability at elevatcd temperatures comprising a mixture of major proportion of ammonium perchlorate, a minor proportion of a polymeric fuel binder composition and at least one solid compound capable of decomposing into ammonia at a temperature below 200 C. selected from the group consisting of ammonium carbonate, ammonium bicarbonate and urea.

6. The composition of claim 5 wherein said compound comprises ammonium carbonate.

7. The composition of claim 5 wherein said compound comprises ammonium bicarbonate.

8. The composition of claim 5 wherein said compound comprises urea.

9. An ammonium perchlorate composition having improved thermal stability at elevated temperatures comprising a mixture of ammonium perchlorate and at least one solid compound capable of decomposing into ammonia at a temperature below 200 C. selected from the group consisting of ammonium carbonate, ammonium bicarbonate and urea.

10. The composition of claim 9 wherein said compound comprises ammonium carbonate.

11. The composition of claim 9 wherein said compound comprises ammonium bicarbonate.

12. The composition of claim 9 wherein said compound comprises urea.

References Cited in the file of this patent UNITED STATES PATENTS 792,511 Frank June 13, 1905 1,273,477 Given July 23, 1916 2,997,501 Shino et a1. Aug. 22, 1961 3,002,830 Barr Oct. 3, 1961 3,024,595 Helvenston et al Mar. 13, 1962 

5. AN AMMONIUM PERCHLORATE-CONTAINING PROPELLANT COMPOSITION HAVING IMPROVED THERMAL STABILTIY AT ELEVATED TEMPERATURES COMPRISING A MIXTURE OF MAJOR PROPORTION OF AMMONIUM PERCHLORATE, A MINOR PROPORTION OF A POLYMERIC FUEL BINDER COMPOSITION AND AT LEAST ONE SOLID COMPOUND CAPABLE OF DECOMPOSING INTO AMMONIA AT A TEMPERATURE BELOW 200*C. SELECTED FROM THE GROUP CONSISTING OF AMMONIUM CARBONATE, AMMONIUM BICARBONATE AND UREA. 